Modified llgs Linux memory region info to use new ProcFileReader.

NativeProcessLinux now uses ProcFileReader to parse /proc/{pid}/maps
line by line.  It now builds a vector of entries up front
rather than parsing until the target memory region is found.
This is in preparation of caching the result once process stop
ids are implemented.

Completes ticket: https://github.com/tfiala/lldb/issues/27